There are a number of applications in which transducers are used to detect incoming radiation. One application is for ranging and obstacle avoidance for a mobile robot, such as described in U.S. Pat. No. 4,701,893 by Muller et al. For the mobile robot described by Muller et al., twenty-four ultrasonic transducers are arranged in a ring about the robot. The large number of transducers is required to provide 360 degrees of coverage, because each transducer transmits pulses of radiation having a narrow beam, that is, a narrow wavefront pattern. In addition to being narrow, the beam has a number of nodes or null zones as little as fifteen degrees from the main beam axis. Detection is poor in these null zones.
Manufacturing a device having a large number of sensors entails more than simply the cost of many transducers. Each transducer requires accompanying circuitry to drive it and to process echo signals developed by the transducer when echo pulses are returned. The sensors consume power and add additional weight which are significant handicaps for devices operated by batteries.
Even where cost, weight and power consumption are not a factor, a large number of sensors would interfere with each other if operated simultaneously. In the mobile robot having twenty-four sensors, for example, the sensors are operated in a number of different banks of sensors; in other words, only several of the sensors are operating at a time. Therefore, only small areas of the entire environment of the robot can be monitored at a time.